Maintaining good image quality over time is dependent on the design aspects of the monitor, and the ability to regulate and control the factors that influence stability. The four key factors are:
- Stabilizing the black level
- Controlling the white level
- Regulating the gamma function of the CRT
- Controlling phosphor degradation.
Black-level stabilization, or cut-off control, is generally accomplished by having an internal closed-loop circuit that monitors and regulates drive circuitry to keep electrical current levels stable and consistent. Advances in digital circuitry have enabled manufacturers to design more stable regulating circuitry than was possible with analog- controlled designs. Small changes in the black level, or brightness, of a monitor can have very noticeable impact on the white level, and subsequently on image quality.
In most cases the use of inadequate black-level stabilization circuitry will result in a decrease in monitor brightness over time. Poor stabilization in this area is generally visualized over a short period of time, and requires frequent recalibration.
Another source of overall brightness reduction and low luminescence, particularly over a longer period of time, is the inability to control the maximum luminance. This was commonly seen in older oxide-cathode display devices. Nearly all high-resolution monitors produced today utilize a dispenser cathode design.
As the name implies, a dispenser cathode dispenses needed oxide material over the lifetime of the monitor, and can maintain the required contrast of the monitor for a much longer time. Typically, there is enough oxide material in a dispenser cathode to maintain monitor performance for eight to ten years.
Once the brightness and contrast levels have been stabilized and controlled, the next step is regulating the CRT’s gamma function, a process not to be confused with gamma correction. After the monitor's brightness and contrast levels have been set to the desired points during calibration, the natural response model of the CRT is measured, and the video card’s drive levels corrected. The purpose of gamma correction is to achieve a desired gamma response model at the face of the monitor.
The monitor must also be stable, with no drift or deviation of the natural gamma response model, which would negatively affect image quality. This regulation is totally a function of the natural physical design of the CRT itself. However, a well-designed and consistently built CRT will maintain this gamma function over time. Manufacturers can provide data regarding the natural gamma of their products, and their lifetime performance history.
Phosphor degradation is the most difficult area to control. The more a monitor is used, and the more often the phosphor is excited, the faster it will degrade. This was the premise for developing screen savers, in order to save life of the CRT.
However, much research has evaluated the lifetime characteristics of different types of phosphors. It is generally accepted that P45 phosphor has a much slower degradation rate than other common grayscale phosphors, like P104. The efficiency of P45 is less, therefore the designer has to be a little more creative in being able to meet the luminance requirements, but P45 offers tremendous benefits in extending monitor lifetime.
The combination of these design choices makes it possible to have a monitor that is self-regulating and can maintain its image quality over time. A properly designed high-resolution grayscale monitor can often maintain excellent image quality for as long as six to eight years, with minimal degradation in performance. Once design elements have been incorporated to ensure stability, the issue of reliability can be addressed.
Next page: Monitor reliability
